Study On The Reduction Of Cr(?) By Shewanella Oneidensis MR-1 And Its Mechanism

Chromium is an element widely found in nature.The toxicity of different forms of chromium is very different and the toxicity of hexavalent chromium is far greater than the toxicity of trivalent chromium.The reduction process of hexavalent chromium can reduce the toxicity of available chromium.A small amount of hexavalent chromium in the environment can be reduced to trivalent chromium by the redox reaction in the natural environment,and the existence of a large amount of chromium pollution can only be managed effectively through artificial intervention.In recent years,research on chromium morphology had become a hot topic,and the widely accepted method was microbial reduction.In this paper,a batch of strains with Cr(?)tolerance and Fe(?)reduction function were cultured under laboratory conditions to explore the reduction mechanism of Cr(?)and investigate the effect factor of Cr(?)removal.The main research content and results were as follows:The reduction of Cr(?)by Shewanella oneidensis MR-1 was under LB culture conditions and its influencing factors.The results showed that the bioreduction of Cr(?)at different initial concentrations was closely related to the tolerance of bacterial strains to chromium.Cr(?)at low concentration had little effect on the strains growth.Cell growth was inhibited at high Cr(?)concentration,thereby inhibiting the reduction of Cr(?);the reduction of Cr(?)increased with increasing the amount of strains inoculum.The optimum p H of the strains growth was neutral,and the weakly alkaline environment was more conducive to the Cr(?)reduction than the acidic environment.The increasing the amount of AQDS accelerated the rate at which Cr(?)was completely reduced.The results of SEM-EDS and XPS analysis showed that Cr(?)and Cr(?)existed on the surface of cells after treatment with Cr(?)for 5 d,which confirmed that Cr(?)reduction by S.oneidensis MR-1 was at the same time also accompanied by a small amount of Cr(?)adsorption.Cr(?)was reduced by S.oneidensis MR-1 with Fe(OH)3 and its influencing factors,and the mechanism of the strain after reduction was characterized by SEM,EDS and XPS.When Cr(?)was added to the anaerobic system of S.oneidensis MR-1 and Fe(?),it is rapidly reduced.S.oneidensis MR-1 can directly reduce Cr(?),but the efficiency of direct reduction is reduced and it is limited.5 mg/L Cr(?)was completely removed in 2 d,which was 1 d faster than the original;and in 20 mg/L Cr(?),Cr(?)in the experimental group with Fe(?)was completely removed at 7 d.The experimental group without Fe(?)added only reached 88% in 7 d.The addition of iron can effectively promote the reduction of Cr(?)by microorganisms,but the effect of dosing on the removal efficiency is less obvious,because during the reduction of Cr(?),iron promotes S.oneidensis MR-1 to Cr(Recovery of VI)can be recycled.Scanning electron microscope(SEM)showed that the addition of Cr(?)had some toxic effects on the iron-reducing bacteria.Transmission electron microscope(TEM)-energy disperse spectroscopy(EDS)showed that there was chromium on the surface of S.oneidensis MR-1.The results of XPS show that Cr2O3 or Cr(OH)3 precipitates were formed during the reduction,and Fe(II)and Fe(?)were also present in the system.Thus,Cr(?)reduction may provide a useful pathway for the detoxification of chromium.Cr(?)was reduced by S.oneidensis MR-1 in soil environment.The effects of Cr(?)concentration,different forms of iron and organic acids on Cr(?)reduction by S.oneidensis MR-1 were studied in soil.Experiments showed that the suspension of S.oneidensis MR-1 bacteria accelerated Cr(?)reduction in the soil and compared the Cr(?)reduction with different forms of Fe(?)magnetite,hematite,and ferric chloride.As shown in Fig.3-18,the Cr(?)reduction effect was better when ferric chloride was added,and the concentration of Cr(?)in the soil was reduced to 65.2 mg/kg,but the treatment group containing magnetite and hematite was added.The concentration of Cr(?)in the soil was only reduced to 162 mg/kg and 192.2 mg/kg,indicating that the effect of Cr(?)reduction was best when ferric chloride was added,followed by magnetite,and the hematite effect was the highest.Poor;this experiment added citric acid,malic acid and oxalic acid and was mainly to detect the effect of organic acids on Cr(?)reduction by microorganisms.Effect of each organic acid on Cr(?)reduction by S.oneidensis MR-1 was as follows: citric acid> malic acid> oxalic acid.